This invention relates to stacked, optically coupled etalons and to methods of making and using them, as well as to devices incorporating such stacked, optically coupled etalons.
Etalons are ubiquitous in optical systems, such as optical sensors, optical communication systems, etc. The basic Fabry-Perot etalon can be designed and produced to have a sharp response at resonant frequencies, which makes them suitable as optical filters such as bandpass filters. They also give a variable amount of dispersion, and so have been suggested for possible use as dispersion compensators. Thus Fabry-Perot etalons are a basic building block in a number of different optical elements, i.e., in optically functional components or devices. Such devices may be active or passive and may be employed in a system (or adapted to be employed in a system) to pass or transmit a selective wavelength or band of wavelengths or periodic set of wavelength bands. Exemplary optical elements in which etalons are used include optical sensors, and filters, e.g., band pass filters, single channel filters, and other wavelength selective filter devices such as wavelength division multiplexers, and dispersion compensators and other components of optical communication systems.
Etalons typically comprise precisely parallel selectively transmissive surfaces such as thin films, i.e., partially reflective mirrors or surfaces on opposite sides of an integral number of half waves distance or gap between them, forming the etalon""s cavity. The thin film and cavity characteristics determine the optical properties of the etalon. That is, the spectral characteristics of the etalon are generally determined by the reflectivity of the mirrors or surfaces and by the optical thickness of the cavity length. Such etalons have long been produced, for example, by sputter deposition of film stacks of alternating layers of materials, i.e. a high refractive index material alternating with a low refractive index material, to form a mirror coating, which is transmissive of selected wavelengths.
Two such mirror coatings sandwich a sputter-deposited cavity layer between them. Sputtering or other physical vapor deposition of the relatively thick cavity layer is time consuming and, therefore adds substantial time and cost to the production of such etalons. The result is undesirably high cost for production for such etalons.
It has long been a recognized problem in this industry, that producing etalons having desired properties can be difficult and expensive. In addition, there are industry-recognized problems associated with producing structurally robust etalons having desired, precise optical properties. Prior known etalons have employed various designs, such as the etalons used in the interferometric optical devices of U.S. Pat. No. 6,125,220 to Copner et al. In the interleaver/de-interleaver devices of Copner et al, two glass interferometric end plates are separated by a spacer region where the etalon is formed. The spacer region is an air gap having a predetermined dimension. In adjustable Fabry-Perot devices, such as those disclosed in U.S. Pat. No. 5,283,845 to Ip, tuning of the center wavelength of the spectral passband of an etalon is achieved by varying the effective cavity length (spacing) between two end plates carrying thin film reflectors. More specifically, in Ip a piezo actuator is used, extending between the two end plates. By varying the electric power applied to the piezo actuator, the axial length of the actuator can be varied, and thus the gap between the end plates varied. As alternatives to piezo-electric actuators, the tuning mechanism may include liquid crystals, temperature, pressure, and other mechanisms. It is a disadvantage that adjustable etalons as in Ip involve considerable assembly complexity and cost. Also, maintaining strict parallelism between the end plates can present additional difficulties.
The prior known optical etalons, as noted above, fail to fully meet the needs of many applications, especially for optical elements intended for optical communication systems, precision sensors, etc.
It is an object of the present invention to provide stacked, optically coupled etalons addressing some of the deficiencies of the prior known technologies. It is a particular object of at least certain preferred embodiments, to provide stacked, optically coupled etalons and methods of making same, and optical elements incorporating such stacked, optically coupled etalons. Additional objects and aspects of the invention and of certain preferred embodiments of the invention will be apparent from the following disclosure and detailed description.
In accordance with a first aspect, an optical element comprises multiple Fabry-Perot etalons stacked and optically coupled. The etalons may be placed in optical contact, i.e. the thin film coatings of an etalon in direct and substantially continuous surface-to-surface contact with the thin film coatings of an adjacent etalon in the stack, or may be placed in contact using one or more bonding layers optically coupled with each other between adjacent etalons.
As used herein, a bonding layer is any layer of bonding material on a surface of an etalon and used to physically attach that etalon to an adjacent etalon. The bonding layer optionally is in the light path through the stacked etalon and serves also to optically couple the adjacent etalon. In such embodiments, preferably the thickness of the bonding layer is equal to an odd number of quarter wavelength optical thickness (QWOTs). The bonding layer may comprise any of numerous materials suitable for bonding etalons together including, but not limited, to adhesives, fritted glass, etc, or other suitable materials. In other embodiments, the bonding layer is omitted and optical contacting is used to attach an etalon to an adjacent etalon. Optical contacting can result in optically zero thickness, e.g. near zero thickness or approximately zero QWOTs, between adjacent etalons.
In accordance with another aspect, one or more etalons of the stacked, optically coupled etalons, preferably each etalon, comprises a bulk optic having first and second parallel, selectively transparent surfaces. The bulk optic comprises a solid optically transparent body (at the wavelengths of interest) and optionally comprises a wedge correcting coating (referred to here generally as a xe2x80x9cwedge coatingxe2x80x9d) and/or a thickness-adjustment layer on at least one of the two surfaces of the optically transparent body. The wedge coating, further described below, establishes high precision parallelism of the selectively transparent surfaces of the etalon. The thickness of the bulk optic (including any wedge coating), i.e., the dimension between the selectively transparent, parallel surfaces, defines the cavity spacing. Preferably, the bulk optic, including the wedge coating, will typically have an optical thickness equal to an integral number of half waves for the wavelength of interest. In preferred embodiments the selectively transparent surfaces are thin film mirror coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies. Preferably, the thin film coatings comprise a continuous uniform thickness metal film.
If a wedge coating is used, the thickness of the wedge coating varies progressively across the etalon. That is, the thickness of the wedge coating, viewed in cross-section in at least one plane orthogonal to the parallel, selectively transparent surfaces of the etalon, has a thickness that increases (or decreases in the opposite direction) continuously, typically approximately linearly, to compensate for non-parallelism, or xe2x80x9cwedgexe2x80x9d, in the underlying body of the bulk optic. As described further below, the bulk optic can be diced from a wafer on which a wedge coating and the two thin film coatings have been deposited by magnetron sputtering, ion beam sputtering or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For an etalon intended for use as one of multiple stacked etalons in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm. Low wavefront error can be understood in this context to mean that the thickness of the bulk optic, i.e., the distance between the two opposite surfaces of the bulk optic, is substantially linearly variable and, hence, controllable or correctable by a wedge coating in accordance with the present disclosure. Preferably, for stacked, optically coupled etalons suitable for use in optical communication elements, the wedge coating brings parallelism of the opposite surfaces of each bulk optic body from the 1 to 2 arc seconds of wedge mentioned above to less than 0.1 arc seconds, most preferably less than 0.01 arc seconds.
It is a significant advantage that each etalon of the stacked, optically coupled etalons disclosed here can employ a bulk optic, comprising the optically transparent body and the wedge coating, to define the cavity spacing of the etalon. Substantial cost savings and production simplification can be realized in accordance with at least certain preferred embodiments of the stacked, optically coupled etalons. Further, robust and accurate stacked, optically coupled etalons can be achieved using production techniques whose application will be readily understood by those skilled in the art given the benefit of this disclosure.
In accordance with a method aspect of the present disclosure, the wedge coating is deposited onto the optically transparent body of the bulk optic by physical vapor deposition, e.g. magnetron sputtering or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical 6 inch or larger substrate wafer used in the production of optical filters) not spinning during deposition. Preferably the coating is a low defect coating to allow optical contact bonding or other optical coupling of the stacked etalons. Deposition can be otherwise in accordance with known techniques, whose applicability and manner of implementation will be within the ability of those skilled in the art given the benefit of this disclosure. The substrate is oriented at an angle to the target or otherwise arranged to receive progressively different deposition rates from one edge to the opposite edge. The target is preferably an elongate source to provide a coating with a roughly linear profile. Preferably, the target is tangential to the circumference of the substrate and is about four times as large as the substrate, e.g. for a 6 inch diameter substrate the target can be about 20 inches long. The thinnest point of the substrate is positioned closest to the target or otherwise oriented or favored to have the fastest rate of deposition. Conversely, the thickest point is positioned or oriented to have the lowest deposition rate. Thus, the sputtered material will deposit fastest and, therefore, the most heavily, i.e., the thickest, where the bulk optic was thinnest, with progressively thinner deposition toward the area where the least was wanted. Since the substrate is not spinning and is oriented or arranged as just described, the thickness of the resulting wedge coating will change progressively (hence the term xe2x80x9cwedgexe2x80x9d), with the change in the thickness of the wedge coating being opposite that of the underlying body. The net effect is that the thickness of the bulk optic is substantially uniform over all or a large portion of its area. Where the bulk optic is prepared in the form of a typical substrate wafer, thin films can then be deposited to complete the etalon (subject to any further production or packaging steps etc.) in the same or a different sputter deposition chamber. Deposition of a wedge coating and thin films on another, second surface of the bulk optic may be performed in accordance with the embodiments described here.
In accordance with another aspect, one or more of the etalons contained within stacked, optically coupled etalons, most preferably all of the stacked etalons, each comprises a thickness-adjustment layer of substantially uniform thickness on at least one of the two surfaces of the optically transparent body. As used herein, substantially uniform thickness means the thickness of the layer across the surface of the bulk optic is approximately constant. The thickness-adjustment layer, further described below, establishes the desired thickness of the bulk optic cavity of the etalon. The bulk optic is a solid, optically transparent (at the wavelengths of interest) body whose thickness, i.e. the dimension between the selectively transparent, parallel surfaces, including the thickness-adjustment layer, defines the cavity spacing. In particular, the bulk optic, including the thickness-adjustment layer, will typically have an optical thickness equal to an integral number of half waves at the same or all points for the wavelength(s) of interest. In preferred embodiments, the selectively transparent surfaces are Fabry-Perot thin film coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies. As described further below, the bulk optic can be diced from a wafer on which a thickness-adjustment layer and the two Fabry-Perot thin film coatings have been deposited by magnetron sputtering, ion beam sputtering, or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For stacked, optically coupled etalons intended for use as one of a plurality of stacked etalons in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm.
It is a significant advantage that the etalons of the stacked, optically coupled etalons disclosed here can employ a bulk optic, comprising the optically transparent body and the thickness-adjustment layer, to define the cavity spacing of the etalon. Substantial cost savings and production simplification can be realized in accordance with at least certain preferred embodiments of the stacked, optically coupled etalons. Further, robust and accurate stacked, optically coupled etalons can be achieved using production techniques whose application will be readily understood by those skilled in the art given the benefit of this disclosure.
In accordance with a method aspect of the present disclosure, the thickness adjustment layer is deposited onto the optically transparent body of the bulk optic by physical vapor deposition, e.g. magnetron or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical 6 inch or larger substrate wafer used in the production of optical filters) spinning during deposition in accordance with known techniques. Preferably the coating is a low defect coating to allow optical contact bonding or other optical coupling of the stacked etalons. Deposition can be otherwise in accordance with known techniques, whose applicability and manner of implementation will be within the ability of those skilled in the art given the benefit of this disclosure. The net effect is that the thickness of the bulk optic is substantially uniform over all or a large portion of its area. Where the bulk optic is prepared in the form of a typical substrate wafer, thin films can then be deposited to complete the etalon (subject to any further production or packaging steps etc.) in the same or a different sputter deposition chamber. From the disclosure above of optical elements comprising a bulk optic and a wedge coating as the cavity of the optical element, it will be apparent to those skilled in the art that embodiments of the present invention comprising a bulk optic with a thickness adjustment layer and no wedge coating can be expected to have non-parallel surfaces carrying the optical thin films. The consequent impact on optical properties of the optical element may be addressed in accordance with suitable techniques, such as tilting of the optical element in use. Selected area of the optical element may provide optical properties meeting product specification for an intended application.
In accordance with another aspect, each etalon of the stacked, optically coupled etalons, in accordance with the above disclosure, further comprises a wedge coating and a thickness-adjustment layer. As noted above, the properties of an etalon will depend, in part, on the cavity spacing. Thus, in accordance with certain preferred embodiments, the thickness of the bulk optic is adjusted, typically after deposition of the wedge coating, by follow-on deposition of a thickness-adjustment layer. In accordance with a further method aspect of the present disclosure, the thickness-adjustment layer is deposited by physical vapor deposition, most preferably magnetron sputtering or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical substrate wafer used in the production of optical filters) spinning during deposition in accordance with known techniques. In accordance with certain highly preferred embodiments, the wedge coating and then the thickness-adjustment layer are deposited onto the bulk optic (again, either alone or as part of a larger substrate) in a single continuous step. The wedge coating is deposited as described above, by deposition onto the bulk optic substrate not spinning in the vacuum chamber. Upon completion of the wedge coating, the bulk optic substrate is caused to commence spinning without interruption in deposition. Deposition is, therefore, continuous for the wedge coating and the thickness-adjustment layer. To control the optical thickness of the cavity formed by the bulk optic, the thickness can be precisely monitored during deposition of the thickness-adjustment layer. Given the benefit of this disclosure, it will be within the ability of those skilled in the art to employ suitable techniques for monitoring and controlling the thickness of the bulk optic as the thickness-adjustment layer is being deposited. Suitable optical monitoring techniques are known, including, for example, that disclosed in U.S. Pat. No. 6,110,337 to Sullivan et al., entitled Sputtering Method and Apparatus with Optical Monitoring, the entire disclosure of which is hereby incorporated by reference.
In accordance with another aspect, additional cavities can be deposited on either surface of a bulk optic etalon, suitable for use in stacked, optically coupled etalons, by suitable deposition techniques, such as, for example, ion beam sputtering, magnetron sputtering, etc. That is, additional cavities can be formed on the bulk optic etalon. More specifically, following deposition of a suitable transition layer overlying the thin film stacks on either surface of the bulk optic etalon, a cavity can be deposited by, for example, sputtering a first suitable thin film coating or stack, such as a sequence of alternating H/L/H film, followed by deposition of a cavity film, followed by deposition of a second thin film coating or stack which preferably is substantially identical to the first thin film coating. Similarly, yet one or more such thin film etalons can be deposited thereafter employing suitable odd QWOT transition layers between, in accordance with thin film principles well known to those skilled in the art. Thus, the advantage of improved optical performance characteristics of a multi-cavity device can be achieved, incorporating a bulk optic etalon of the type disclosed above together with additional thin film etalons unitary therewith by depositing such thin film etalons thereon using sputtering or other suitable deposition techniques. Any suitable materials disclosed above, and other suitable materials known to those skilled in the art, may be used for the thin films of the thin film etalons including but not limited to Ta2O5, ZrO2, TiO2, Al2O5, SiO2, and MgF. The multi-cavity deposition product, described above, is suitable for use, generally together with or in place of the single cavity embodiments of the bulk optic etalons disclosed here.
In accordance with certain embodiments as disclosed above, the stacked, optically coupled etalons described above may be bonded to each other. Each etalon comprises any or all of the components listed above including, but not limited to, a bulk optic, optionally comprising a wedge coating and/or a thickness-adjustment layer, and thin film mirror coatings on the surfaces of the bulk optic. Between adjacent etalons may be a layer of bonding material that acts to optically couple the etalons together. The thickness of the bonding layer is preferably equal to an odd number of QWOTS. This bonding layer may comprise an adhesive, such as an epoxy, e.g. the epoxies available from Epoxy Technology, Billerica, Mass., such as EPO-TEK 353 ND, an adhesive optionally having a monolayer of small beads embedded in the adhesive to facilitate precise spacing of the adjacent etalons, fritted glass or any other composition or material that may be deposited to an odd number of QWOTs and is capable of bonding the etalons together.
In accordance with another aspect, a method for production of the stacked, optically coupled etalons is disclosed. Each etalon may comprise the components discussed above, including but not limited to, a bulk optic, optionally comprising a wedge coating and/or a thickness-adjustment layer, and thin film coatings on the surfaces of the bulk optic. In accordance with preferred embodiments, multiple etalons can be formed simultaneously be depositing thin film mirror coatings onto both planar surfaces of a planar wafer formed of silica, or any other suitable optical glass or suitable optical material. Such deposition preferably is by physical vapor deposition, as disclosed above. Optionally, the wafer is first planarized and coated with a wedge coating and/or a thickness-adjustment layer. After a selectively transparent thin film mirror coating is deposited on each side of the wafer, the wafer can be cut or diced into multiple smaller pieces, some or all of which will be functional as an etalon for a particular wavelength. Optically matched etalons can then be stacked and joined as disclosed above. Alternatively, one such wafer can be joined to a second such wafer, wherein at least in some overlying areas the wafers are optically matched to each other. Two or more wafers can be stacked and joined in this manner. The stacked wafers are then cut or diced to provide multiple functional units of stacked, optically coupled etalons. Optionally, prior to dicing the wafer into individual etalons or into intermediate size xe2x80x9ccouponsxe2x80x9d or the like, a bonding or joining material can be deposited onto one or both sides. Preferably such deposition is by sputter deposition, or other physical vapor deposition method, using optical monitoring to achieve proper thickness. Typically, the thickness of the joining or bonding layer will be an odd number (i.e. an odd integer) of QWOTs. In certain preferred embodiments further discussed below, the bonding layer will remain in the light path between adjacent etalons. In such embodiments, the bonding layer is formed of optically transparent material. In certain preferred embodiments, the bonding layer is selectively removed from the light path, e.g., by chemical etching, leaving an annulus or multiple discrete xe2x80x9cfeetxe2x80x9d to serve as stand-offs between adjacent etalons. In such embodiments the material used to form the xe2x80x9cfeetxe2x80x9d need not be optically transparent.
In accordance with certain preferred embodiments, the etalons are joined by depositing an optically transparent flitted glass layer on a surface of etalons. The fritted glass is preferably deposited using a vapor deposition process to a controlled thickness. The fritted glass layer may be deposited onto substrate wafers as disclosed above or onto individual etalons. Two or more wafers can be stacked or, alternatively, individual coupons or individual etalons can be stacked after being coated with fritted glass. After being placed in physical contact, the etalons are heated to the melting or softening point of the fritted glass, bonding the stacked etalons together. Preferably, after the etalons are joined, the thickness of the fritted glass layer between them is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are joined and stacked.
In accordance with certain preferred embodiments, the etalons are joined using xe2x80x9cfeet.xe2x80x9d As used herein, xe2x80x9cfeetxe2x80x9d are projections from opposite surfaces of adjacent etalons. xe2x80x9cFeetxe2x80x9d can be formed of fritted glass or other suitable materials able to provide a structurally stable stacking of the adjacent etalons to each other. After adjacent etalons are positioned in contact, heat can be applied to sufficiently melt or soften the xe2x80x9cfeetxe2x80x9d so as to join the etalons to each other. Preferably, after joining the etalons, the distance between the etalon surfaces is equal to an odd number of QWOTs.
In accordance with certain preferred embodiments, adjacent etalons are joined using a layer of epoxy or other suitable adhesive. The adhesive can be applied using any suitable technique. The adhesive may be deposited onto wafers, coupons, or onto individual etalons. Two or more wafers, two or more coupons, or individual etalons are stacked in physical contact after the adhesive is applied. After being placed in physical contact, the adhesive layer bonds the etalons forming stacked, optically coupled etalons. Preferably, after the etalons are joined, the thickness of the epoxy layer is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are coupled and stacked. In accordance with certain preferred embodiments, the etalons are joined using adhesive comprising optically transparent beads sized to function as a monolayer to assure a controlled spacing between the etalons. The small bead/adhesive mixture may be deposited onto wafers, coupons or onto individual etalons. Wafers, coupons, or etalons are placed in physical contact after being coated with the bead/adhesive mixture. After being placed in physical contact, the bead/adhesive mixture bonds the etalons forming stacked, optically coupled etalons having a uniform separation. Preferably, after the etalons are joined together, the thickness of the bead/adhesive layer is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are coupled and stacked.
In accordance with preferred embodiments, the etalons may be coupled together using optical contact. To accomplish optical contact bonding, adjacent etalons are placed in physical contact with each other. Etalons are prepared as described above. The etalons are then placed into physical contact and, while not wishing to be bound by theory, it is presently understood that adjacent etalons in optical contact with each other are held together by electrostatic forces between the contacting surfaces of the etalons. Therefore, after optical contacting adjacent etalons there is approximately near zero thickness between the adjacent etalons, e.g. near zero QWOTs. For certain preferred embodiments, the selectively transparent thin film mirror coatings of the stacked etalons are formed of alternating films of high refractive index material and low refractive index material. Preferably, the outermost film of the mirror coating of one etalon is the same (i.e., either low or more preferably high refractive index) as the outermost film of the mirror coating of the adjacent etalon. The transition layer between them preferably has the opposite refractive index. Thus, if the two outermost films facing each other are high refractive index, then an air spacer or other low refractive index material preferably fills the odd QWOT space between the two etalons. Likewise, if the mirror coatings finish with low refractive index materials, then the joining transition layer between the two etalons should have a high refractive index.
Alternatively, the thickness of the outermost layer of the thin films may be reduced such that, when combined together they form a joining transition layer of refractive index opposite to that of the next film on either side. In other preferred embodiments, the etalons may be held in a stacked position using a coupling member such as a metal jacket a sleeve, etc. Should a coupling member be used, any space that exists between the etalon surfaces is preferably equal to an odd number of QWOTs and may be filled with air or other suitable materials.
In accordance with another aspect, the stacked and optically coupled etalons disclosed here may be incorporated into optical systems. The stacked, optically coupled etalons can be directly optically coupled, as the term is used here, when they are optically coupled, i.e. are in the same optical path, and furthermore are in optical contact or are otherwise in physical contact with each other and/or mounted to each other (e.g. by bonding material in or out of the optical path) or mounted together in the same housing or by the same fixture. An air space may be separating the stacked, optically coupled etalons or the stacked, optically coupled etalons may be in direct surface-to-surface contact. Similarly, optical components in an optical system comprising the stacked, optically coupled etalons disclosed here, that are directly optically coupled in accordance with the present disclosure, preferably have no intervening optical components performing substantial channel filtering or like optical operation on any passed signals. In particular, any signals passed by the stacked, optically coupled etalons, or other optical components in the optical system, arrive at a second etalon or second stacked, optically coupled etalons, or other optical component, without any intervening wavelength filtering optical operations to add or drop passbands or like operations. One skilled in the art, given the benefit of this disclosure, will be able to design and assemble optical system comprising directly optically coupled and stacked, optically coupled etalons and other optical components described here.